To accommodate rapidly increasing traffic in recent years, there is an increasing demand in the optical communication field for expansion of transmission capacity toward 100 Gbps or 400 Gbps per channel, as well as for reduction in power consumption. Under such circumstances, compact design operable in a radio frequency band is desired for optical components such as optical modulators used in optical communications frontend circuits.
There are several types of electro-optic modulators operable at a radio frequency (RF), including lithium niobate (LiNbO3) modulators, indium phosphate (InP) modulators, and silicon (Si) waveguide modulators. Among them, LiNbO3 modulator (referred to as a “LN modulator”) is currently mainstream in the optical modulator markets, from the viewpoint of the quantity of insertion loss, transmission characteristics, and controllability. In a typical LN modulator, titanium (Ti) is diffused in a LN substrate to fabricate optical waveguides. However, the modulation efficiency of LN modulators is insufficient due to poor light confinement. Besides, the chip length becomes 5 cm or more in order to guarantee the half-wavelength voltage Vπ of the modulator.
A configuration with ridge waveguides formed on an LN substrate is proposed to enhance light confinement. See, for example, Patent Documents 1 to 3 listed below. FIG. 1 is a cross-sectional view of an optical modulator with ridge-type optical waveguides 131a and 131b. When a Z-cut LN substrate 120 is used, a signal electrode S is provided over the ridge waveguide via a buffer layer 14.
To enhance light confinement by means of a ridge waveguide on a Z-cut LN substrate, the height and the width of the waveguide become about 1 μm. The width of the signal electrode S provided on the ridge waveguide also becomes as narrow as about 1 μm. In general, the larger the cross sectional area of the electrode, the less the electrical attenuation, and RF characteristics can be improved. In order to increase the cross-sectional area of the signal electrode S under the configuration of FIG. 1, the height “h” of the signal electrode S will be 10 μm or more. It is difficult to form a signal electrode with such a high aspect ratio. Even though a high-aspect electrode could be fabricated, the electrode has a tendency to fall down and it is difficult to efficiently apply a voltage to the ridge waveguide. A compact design for an optical modulator with satisfactory modulation efficiency and RF characteristics is desired.